As methods of forming a dense-structure coating film (several micrometers or several tens of micrometers (μm)) on the surface of a support, there are gas-phase methods and liquid-phase methods. Examples of gas-phase methods may include electrochemical vapor deposition (EVD), chemical vapor deposition (CVD), sputtering, ion beam method, electron beam method, and the like. However, each of the gas-phase methods has at least one disadvantage, such as requirement of expensive manufacturing equipment, starting material restrictions, difficulty in fabricating a thick specimen attributable to low thin film growth rate, insufficient adhesion between a coating film and a substrate, stripping of a coating film due to residual stress, limitation in size of a specimen, and the like. For this reason, liquid-phase methods, which are relatively easily carried out compared to gas-phase methods, are frequently used. Particularly, examples of liquid-phase methods may include sol-gel process, slip coating, slurry coating, spin coating, dipping, electrochemical process, electrophoresis, hydrothermal synthesis, and the like. Among these liquid-phase methods, in the dipping, spin coating, slurry coating including spray coating or sol-gel process, a coating layer is dried or gelled in the early stage because of its low green density, and simultaneously, is greatly contracted. The contraction of a coating layer causes a stress between a support and a coating layer, and this stress becomes more severe in the subsequent sintering process, thereby causing cracking of the coating layer and stripping of the coating layer from the support. As mentioned in the paper (K. Murata and M. Shimotsu, Denki Kagaku, V. 65, No 1, 1997), it is known in this paper that the thickness of a coating layer applied at one time must be adjusted to 1 μm or less in order to prevent the cracking of the coating layer and the stripping of the coating layer from the support. That is, there is a disadvantage in that drying and heat treatment processes must be repeated at least ten times in order to obtain a dense-structure coating layer having a thickness of 10 μm using this method. Further, as mentioned in the paper (T. Ishihara, J. Am. Ceram. Soc., Vol. 79, No. 4, pp 913-19, 1996), the method mentioned in this paper is also disadvantageous in that, in the selection of a substrate in the electrochemical deposition, electrophoresis or hydrothermal synthesis, raw materials having high electrical conductivity must be used.
Here, a dense-structure film is referred to as a film adhered strongly to a support to a sintered density of 30% or more. When the sintered density of such a dense-structure (gas-impermeable (or gas tight) or porous functional) film is 90% or more, it is known that gas scarcely transmits the film. Only this dense-structure film is used in a general electrochemical device or a mechanical or electrical device having functionality.
However, a method of forming a dense-structure film using a semiconductor process, such as CVD, EVD or the like, is known to be unsuitable for manufacturing a large-area support-type coating film, because expensive manufacturing equipment is used, and a lot of processing time is required, thus increasing a total manufacturing cost. Therefore, there has generally been used a method of manufacturing a support-type coating film, including the steps of: dispersing ceramic powder particles in a solution containing an organic binder in a solvent to form a slurry; directly coating a support (ceramic support or metal support) with the slurry using dip coating, spin coating or spray coating; and co-sintering the support coated with the slurry. However, this method is disadvantageous in that only one side of the support is coated, and it is difficult to maintain the uniform thickness and reproducibility of the coating film according to the manufacturing conditions.
Particularly, in the case of a fuel cell, when an electrolyte sheet directly adheres to a support, the green (dried) electrolyte sheet is torn in the step of adhering, or the contraction rate thereof cannot be easily controlled due to the adhesion between the electrolyte to the support during co-sintering, so a dense-structure electrolyte membrane cannot be obtained, and thus it is difficult to manufacture a single cell. Therefore, conventionally, as in a multilayer dielectric chip (MLDC) method, calendar method or conventional patent (manufacture of an anode support-type single cell), there has been used a method of manufacturing a single cell by laminating electrolyte and anode sheets to form a sheet cell and then applying a slurry to the sheet cell. However, this method is also problematic in that, in the process of manufacturing a single cell, the sheet cell must be prepared by separately fabricating the electrolyte sheet and the anode sheet every time and then respectively adhering these sheets to a support, and these sheets adhere to the support using a slurry for adhering, thus causing the non-uniformity of composition and thickness of the sheet cell, and in that the manufacturing cost thereof increases.
Further, a conventional multilayer sheet adhering method has been widely known as a method of manufacturing a multilayer dielectric chip (MLDC). However, this method is problematic in that a large number of tape-cast sheets are required to manufacture a single cell, so the production yield thereof rapidly decreases, thereby increasing the production cost thereof.
Hitherto, in the field of solid oxide electrolysis cells or fuel cells, as proposed in the paper (N. Q. Minh, J. Am. Ceram. Soc., Vol. 76, No. 3, pp 563-88, 1993), it is known that a tape casting method or doctor blade method is widely used in manufacturing a thin plate having a thickness of several micrometers (μm) or several tens of micrometers (μm), but is difficult to manufacture a multilayer specimen including a support and a coating layer made of different materials. In this case, a dense-structure coating film cannot be formed by only a single-component green electrolyte sheet. Therefore, there has been known a method of manufacturing a single cell by adhering (attaching) a cathode green sheet, an electrolyte green sheet and an anode green sheet to form a multi-composite layer and then heat-treating the multi-composite layer or a method of manufacturing a single cell by adhering (attaching) an electrolyte green sheet and a cathode green sheet or an electrolyte green sheet and an anode green sheet to form a double composite layer and then attaching the double composite layer to a support or forming another constituent (anode or cathode). However, in this case, a process of preparing a composite green sheet by attaching several green sheets to each other must be performed. Therefore, this method is also problematic in that the manufacturing cost thereof increases, in that, when the single cell is manufactured by attaching large-area sheets to each other, these sheets are non-uniformly attached, and thus the defective fraction thereof increases, thus decreasing the production yield thereof, and in that, when the single cell is manufactured using roughly-surfaced sheets, the attaching of these sheets becomes difficult, thus rapidly increasing the manufacturing cost thereof.
According to a conventional tape casting method, an electrolyte layer is formed into a green sheet having a thickness of 100 to 200 μm, anode paste is printed on one side of the green sheet, dried and heat-treated to form an anode, and then cathode paste is printed on the other side of the green sheet, dried and heat-treated to form a cathode, thus manufacturing a single cell. In this case, since the electrolyte green sheet serves as a matrix, the manufactured single cell may be an electrolyte support-type single cell. However, this method is problematic in that, as the thickness of the electrolyte layer increases, the internal resistance thereof increases, so operation temperature must be high, and the output performance of the single cell deteriorates, and in that, since the single cell can be operated only when the thickness thereof is 300 μm or less, the strength thereof is relatively low, and thus this single cell is apt to break. Further, this method is also problematic in that it is difficult to manufacture a large-area single cell because the flatness of the electrolyte layer in sintering is poor.
Meanwhile, there has been a method of manufacturing a layered hard sintered body, wherein an electrolyte, an anode and a cathode are respectively prepared by tape casting, and their respective green sheets are formed into a two-layer laminate, that is, an electrolyte-anode laminate or an electrolyte-cathode laminate or are overlapped, pressed and adhered to each other to form an electrolyte-anode-cathode laminate, and then these laminates are simultaneously sintered, thereby obtaining the layered hard sintered body. Even in this case, in order for a single cell to have enough strength to be operated, several sheets of any one of an electrolyte, an anode and a cathode are overlapped and laminated. Here, when several anode sheets are overlapped and laminated, an anode support type SOFC (solid oxide fuel cell) is manufactured, and, when several cathode sheets are overlapped and laminated, a cathode support type single cell is manufactured. In this case, since the thickness of a sintered electrolyte sheet having high internal resistance can be easily adjusted to 5˜50 μm, there is an advantage of manufacturing a low temperature operation type SOFC having lower resistance and generating high power. In this step, a multi-composite green sheet may prepared by a calendar rolling process including tape casting and attaching steps. In the calendar rolling process, even when any one previously tape-cast green sheet (for example, anode) is sequentially tape-cast thereon with other green sheets (for example, electrolyte and cathode), a multi-composite green sheet having the same effect can be manufactured. Finally, it is widely known that such a multi-composite green sheet is heat-treated (sintered) to obtain a sintered body, and this sintered body is screen-printed and dried to manufacture a SOFC. However, in this case, there are many problems in that a large number of green sheets for tape casting is required in manufacturing a single cell, thus decreasing the production yield thereof and increasing the production cost thereof, and in that a large amount of an organic binder is required for tape casting, so a heat treatment cost for removing the organic binder is needed, and environmental pollution, consumption of raw materials, and the like are caused.